Originally posted by xandy Greetings
How this three parameters work together ?
The main relationship in this context is that having a wider maximum aperture allows more light for the autofocus system, which should help it focus faster.
Quote: I'm considering to purchase one of the following; DA 70 or FA 77 or DFA 100 , which of them will give me a fastest focus in low light conditions (to freeze motion) while using with a K100D ?
Normally when people talk about freezing motion, they mean a high shutter speed. A wider aperture will help with that. I think also, though, that you mean getting a focus lock quickly on moving subjects. The answer here is: the focus assist from the built-in flash (blinkyblinkyblinky!) will help a little bit, but in low light, this is hard for any autofocus system, and Pentax by all accounts isn't the fastest available. (Although haven't tested extensively, my K10D it seems slightly faster than my friend's old Canon 10D but much slower than his 30D.) In many situations, your best bet is still going to be to learn to manually focus quickly.